Carrier Aggregation
In the advanced wireless networks developed by members of the 3rd-Generation Partnership Project (3GPP), carrier aggregation is one of the recently introduced ways of increasing per-user throughput for users having good channel conditions and having the capability to receive and transmit at higher data rate. With carrier aggregation (CA), as applied to the fourth-generation 3GPP systems referred to as Long-Term Evolution (LTE) systems, for example, a user can be configured to transmit and/or receive data in two or three (or more) simultaneous bands in the downlink (DL) and/or in the uplink (UL).
FIG. 1 illustrates an example base station (an evolved Node B, or eNB, in the 3GPP terminology for LTE systems) that is capable of operating four different cells at the same time. These cells are operated in different bands, or they could also be operated in the same band. In networks compliant with only earlier versions of the 3GPP specifications, i.e., up to Release 8, only one cell is used for communication between the eNB and the wireless terminal (referred to as a user equipment, or UE, in 3GPP documentation). Note that the term “cell” as used here is closely related to the term “carrier,” and refers to an independent set of radio channels operated in such a way that a UE can obtain service using only one cell. In a carrier aggregation scenario, compatible UEs may receive and transmit data on more than one carrier/cell, depending on how those UEs are configured by the network. The separate carriers in a carrier aggregation scenario are referred to as component carriers (CCs).
CA Cases Based on Different Number of CCs in DL and/or UL
2DL CA (CA with two DL CCs and one UL CC)                FIG. 2 illustrates a scenario in which two of the cells are activated for one UE, which is the initial version of DL carrier aggregation. In this case, the UE is configured to receive in two DL bands simultaneously, while using UL in only one of the bands. The specific UL allocation in this case is arbitrary, meaning that either of the bands can be used for UL transmission.        In carrier aggregation terms, the cell where the UL is allocated for a particular UE is the PCell (primary cell) for that UE, while the other aggregated cell is an SCell (secondary cell). PCell and SCell combinations are UE-specific, in that the network can configure different UEs to have different allocations of PCell and SCells while using the same carriers.        
3DL CA (CA with three DL CCs and one or two UL CCs)                FIG. 3 illustrates a scenario in which three DL carriers, falling in two different frequency bands, are allocated to a UE. As in the 2DL case, the UL can be allocated to any of the bands. Because the carriers are in different frequency bands, this configuration is referred to as inter-band carrier aggregation, similar to 2DL inter-band carrier aggregation.        
2UL CA (CA with two UL CCs and two or three DL CCs)                Unlike the two previous cases, FIG. 4 illustrates a scenario in which UL carrier aggregation is also enabled for the terminal. In this case, only two UL and two DL carrier aggregation is shown. In case of UL carrier aggregation, PCell and SCell definitions are still UE-specific. 2UL carrier aggregation can be combined with carrier aggregation of more than two DL carriers. In essence, there are no restrictions between different UL and DL carrier aggregation to be used simultaneously.Carrier Aggregation Deployment Scenarios        
Depending on the carrier frequency(ies), or depending on the physical eNB deployment, the deployment of CA-enabled systems can be very different.
FIG. 5 illustrates two examples of CA deployment. The left-hand figure shows that two cells designated “f1” and “f2” are co-located and overlaid, but f2 has smaller coverage due to larger path loss. (In the figure, the regions of f2 coverage are shaded, while the larger f1 coverage regions are not.) “f1” and “f2” indicate that the cells are operated on different frequencies, which may or may not be in the same frequency band. The differences in coverage seen in FIG. 5 may be the case, for example, where f2 and f1 are operated on widely separated frequency bands.
In the scenario shown in FIG. 5, only f1 provides comprehensive coverage for the entire region, while f2 is used to improve throughput for those UEs that are adequately covered. Mobility in this scenario is necessarily performed based on f1 coverage. A scenario like that illustrated in FIG. 5 is likely when f1 and f2 are of different bands, e.g., f1={800 MHz, 2 GHz} and f2={3.5 GHz}, etc. In this scenario, it is expected that aggregation is possible between overlaid f1 and f2 cells.
The figure on the right-hand side of FIG. 5 shows a different kind of deployment. In this case, f1 cells provide macro coverage, while f2 Remote Radio Heads (RRHs) are used to improve throughput at hot spots. Again, mobility is performed based on f1 coverage. A likely scenario is that f1 and f2 are of different bands, e.g., f1={800 MHz, 2 GHz} and f2={3.5 GHz}, etc. It is expected that f2 RRHs cells can be aggregated with the underlying f1 macro cells.
UL Configurations
In LTE, the nominal number of resource blocks (RBs, the basic time-frequency radio resource in LTE) is 6, 15, 25, 50, 75 and 100 RBs for channel bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz, respectively. This is shown in table 1, below. However the actual maximum limit for any given UL configuration in practice, in terms of allowed RBs, is pre-defined in the standard for different bands and CA configurations. These limits are designed to ensure that the UE meets one or more pre-defined receiver requirements, such as a requirement for UE receiver sensitivity referred to in LTE specifications as REFSENS. The limits on UL configuration also depend upon the channel bandwidth.
More specifically, the UL configurations specified by the standards for different band and different CA configurations determine the maximum UL transmission block size, in terms of RBs, when a particular CA configuration is used or when a single UL carrier is used. Three representative tables 2, 3 and 4 from the 3GPP specification, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception,” 3GPP TS36.101, v. 12.2.0 (January 2014), available at http://www.3gpp.org, are presented below, and show allowed UL configurations when single UL transmission, or inter-band CA, or intra-band non-contiguous CA, respectively, are used. It can be observed from tables 1-4 that for certain bands, CA, and bandwidth (BW) combinations in CA, the UL configuration (i.e., the maximum number of allowed UL RBs) is severely reduced compared to the corresponding nominal values (i.e., in table 1).
TABLE 1Transmission bandwidth configuration NRBin E-UTRA channel bandwidths for both UL and DLChannel bandwidth BWchannel [MHz]1.435101520Transmission615255075100bandwidth configurationNRB
TABLE 2Uplink configuration for reference sensitivityE-UTRA Band/Channel bandwidth/NRB/Duplex modeE-UTRA1.43DuplexBandMHzMHz5 MHz10 MHz15 MHz20 MHzMode1255075100FDD26152550 501  501FDD36152550 501  501FDD4615255075100FDD561525 251FDD625 251FDD7255075  751FDD861525 251FDD92550 501  501FDD10255075100FDD1125 251FDD12615 201 201FDD13 201 201FDD14151 151 FDD. . .17201 201 FDD1825251 251 FDD1925251 251 FDD2025201 203 203 FDD2125251 251 FDD222550501 501 FDD23615255075100 FDD242550FDD256152550501 501 FDD2661525251 251 FDD2761525251 FDD281525251 251 251 FDD3025251 FDD3165454FDD. . .33255075100 TDD34255075TDD35615255075100 TDD36615255075100 TDD37255075100 TDD38255075100 TDD39255075100 TDD40255075100 TDD41255075100 TDD42255075100 TDD43255075100 TDD4415255075100 TDDNOTE 1:1refers to the UL resource blocks shall be located as close as possible to the downlink operating band but confined within the transmission bandwidth configuration for the channel bandwidth (Table 5.6-1).NOTE 2:For the UE which supports both Band 11 and Band 21 the uplink configuration for reference sensitivity is FFS.NOTE 3:3refers to Band 20; in the case of 15 MHz channel bandwidth, the UL resource blocks shall be located at RBstart 11 and in the case of 20 MHz channel bandwidth, the UL resource blocks shall be located at RBstart 16NOTE 4:4refers to Band 31; in the case of 3 MHz channel bandwidth, the UL resource blocks shall be located at RBstart 9 and in the case of 5 MHz channel bandwidth, the UL resource blocks shall be located at RBstart 10.
TABLE 3Uplink configuration for reference sensitivityE-UTRA Band/Channel bandwidth/NRB/Duplex modeEUTRA CAEUTRA1.4 MHz3 MHz5 MHz10 MHz15 MHz20 MHzDuplexConfigurationband(dBm)(dBm)(dBm)(dBm)(dBm)(dBm)modeCA_2A-29A22550FDD29N/AN/AN/ACA_4A-29A42550FDD29N/AN/AN/ACA_23A-29A23255075100FDD29N/AN/AN/A
TABLE 4Intra-band non-contiguous CA uplink configuration for reference sensitivityAggregatedchannelCAbandwidthUL PCCΔRIBNCDuplexconfiguration(PCC + SCC)Wgap/[MHz]allocation(dB)modeCA_3A-3A25RB + 25RB45.0 < Wgap ≦ 65.01214.7FDD 0.0 < Wgap ≦ 45.0251025RB + 50RB40.0 < Wgap ≦ 60.01213.8 0.0 < Wgap ≦ 40.0251025RB + 75RB35.0 < Wgap ≦ 55.01213.6 0.0 < Wgap ≦ 35.02510 25RB + 100RB30.0 < Wgap ≦ 50.01213.4 0.0 < Wgap ≦ 30.0251050RB + 25RB30.0 < Wgap ≦ 60.01295.1 0.0 < Wgap ≦ 30.0321050RB + 50RB25.0 < Wgap ≦ 55.01294.3 0.0 < Wgap ≦ 25.0321050RB + 75RB20.0 < Wgap ≦ 50.01293.8 0.0 < Wgap ≦ 20.03210 50RB + 100RB15.0 < Wgap ≦ 45.01293.4 0.0 < Wgap ≦ 15.0321075RB + 25RB25.0 < Wgap ≦ 55.0 12106.0 0.0 < Wgap ≦ 25.0321075RB + 50RB20.0 < Wgap ≦ 50.0 12104.7 0.0 < Wgap ≦ 20.0321075RB + 75RB15.0 < Wgap ≦ 45.0 12104.2 0.0 < Wgap ≦ 15.03210 75RB + 100RB10.0 < Wgap ≦ 40.0 12103.8 0.0 < Wgap ≦ 10.03210100RB + 25RB 15.0 < Wgap ≦ 50.0 16116.5 0.0 < Wgap ≦ 15.03210100RB + 50RB 10.0 < Wgap ≦ 45.0 16115.1 0.0 < Wgap ≦ 10.03210100RB + 75RB 5.0 < Wgap ≦ 40.0 16114.5 0.0 < Wgap ≦ 5.03210100RB + 100RB0.0 < Wgap ≦ 35.0 16114.1CA_4A-4ANOTE 6NOTE 7NOTE 80.0FDDCA_7A_7A50RB + 50RB25.0 < Wgap ≦ 50.03210.0FDD 0.0 < Wgap ≦ 25.05010.075RB + 25RB20.0 < Wgap ≦ 50.03210.0 0.0 < Wgap ≦ 20.05010.075RB + 50RB20.0 < Wgap ≦ 45.03210.0 0.0 < Wgap ≦ 20.05010.075RB + 75RB15.0 < Wgap ≦ 40.03210.0 0.0 < Wgap ≦ 15.05010.0100RB + 75RB 15.0 < Wgap ≦ 35.03610.0 0.0 < Wgap ≦ 15.05010.0100RB + 100RB15.0 < Wgap ≦ 30.03210.0 0.0 < Wgap ≦ 15.04510.0CA_23A-23ANOTE 6NOTE 7NOTE 80.0FDDCA_25A-25A25RB + 25RB30.0 < Wgap ≦ 55.01015.0FDD 0.0 < Wgap ≦ 30.02510.025RB + 50RB25.0 < Wgap ≦ 50.01014.5 0.0 < Wgap ≦ 25.02510.050RB + 25RB15.0 < Wgap ≦ 50.01045.5 0.0 < Wgap ≦ 15.03210.050RB + 50RB10.0 < Wgap ≦ 45.01045.0 0.0 < Wgap ≦ 10.03210.0CA_41A-41ANOTE 6NOTE 7NOTE 80.0TDDNOTE 1: 1refers to the UL resource blocks shall be located as close as possible to the downlink operating band but confined within the transmission.NOTE 2: Wgap is the sub-block gap between the two sub-blocks.NOTE 3: The carrier center frequency of PCC in the UL operating band is configured closer to the DL operating band.NOTE 4: 4refers to the UL resource blocks shall be located at RBstart = 33.NOTE 5: For the TDD intra-band non-contiguous CA configurations, the minimum requirements apply only in synchronized operation between all component carriers.NOTE 6: All combinations of channel bandwidths defined in Table 5.6A.1-3.NOTE 7: All applicable sub-block gap sizes.NOTE 8: The PCC allocation is same as Transmission bandwidth configuration NRB as defined in Table 5.6-1.NOTE 9: 9refers to the UL resource blocks shall be located at RBstart = 25.NOTE 10: 10refers to the UL resource blocks shall be located at RBstart = 35.NOTE 11: 11refers to the UL resource blocks shall be located at RBstart = 50.Maximum Power Reduction
Maximum output power (MPR) is defined as an allowed reduction to a UE's maximum output power due to higher order modulation and transmit bandwidth configuration. A general MPR formula is defined in 3GPP specifications for UE operation for different transmission modes, e.g., single UL transmission, 2UL CA transmission, etc. The UE applies the MPR based on UL transmission parameters, e.g., modulation, UL configuration, CA type or configuration, etc.
An Additional Maximum Power Reduction (A-MPR) is defined for certain bands; the A-MPR is allowed to be applied on top of MPR for certain bands. A-MPR is usually defined for specific coexistence requirements, etc. The A-MPR is signaled to the UE by the network node.
Both MPR and A-MPR are used by the UE to comply with one or more radio emission requirements, e.g., out of band emission, spurious emission or additional spurious emission requirements.
Unwanted Emissions
In an ideal case, the transmitter radio is not supposed to transmit anything outside its transmission spectrum. However, due to limitations of practical radio systems, there will be unwanted signals outside the allowed spectrum. The power spectral density (PSD), i.e., the signal power per unit bandwidth, for a UE's transmission depends on transmission power, transmission bandwidth, location of the UL RBs allocated to the UE, etc. The unwanted emissions depend on many issues, e.g., bandwidth of the signal, the transmission power, etc. For larger transmission bandwidth, the unwanted spectrum is also larger compared to smaller transmission bandwidth, when PSD/RB is the same for both cases. Usually for fixed transmission power, with larger transmission bandwidth, the PSD will be smaller, thus the strength of unwanted signals will be smaller for larger transmission bandwidth compared to small transmission bandwidth for fixed transmission power. However, the spread in frequency will also be larger.
The 3GPP specifications for UE operation outline two separate kinds of unwanted emissions, namely (1) Out-Of-Band (OOB) emissions and (2) spurious emissions, both of which are outside the intended channel bandwidth. A schematic diagram illustrating limits for these unwanted emissions is shown in FIG. 6.
Out-of-band emissions are those which fall in a band close to the intended transmission, while spurious emissions can be at any other frequency. The precise boundary between the OOB range and the spurious range is different for different aspects of the LTE specifications.
LTE defines requirements for both types of unwanted emission, with those for spurious emissions being the more stringent. Since different bands have different maximum allowed transmission bandwidths and also different bands have different coexistence requirements, the spurious emission requirements are appended by additional spurious emission requirements in certain bands.
Inter-Modulation Distortion
Inter-modulation distortion (IMD) happens when two or more tones, i.e., distinct signals or carriers, are present in a non-linear device, such as an amplifier in a receiver. In this case, intermodulation products are created. In a single UL transmission, intermodulation (IM) products may happen due to the actual RB allocation and its image being inter-modulated in the receiver. Odd-ordered products are typically of most concern, as these are most likely to fall in or near a band of interest. The strength of an IMD product depends on its order; thus, 3rd-order and 5th-order IMD most often cause receiver degradation. However, r-order IMD can also be detrimental to receiver performance.
In FIG. 7, an example for CA with a 20 MHz primary component carrier (PCC) and a 5 MHz secondary component carrier (SCC) is illustrated. Here, when only one UL is used at 20 MHz (shown at the right hand side of the UL band), then the distance in frequency for IM3, IM5 and IM7 falls as shown in the figure.
Whether IMD falls in a receiver band of the receiver or not depends on the duplex gap, the position of UL and/or DL transmission signals, etc. Depending on IMD in the receive band, several different additional mitigations may be required. For example, inter-band CA between band 4 and band 12 causes harmonics and IMD from B12 to B4, thus, the use of Band 12 with Band 4 requires that additional components to remove the harmonics in the Band 12 receiver. These additional components mean that the Band 12 receiver has a loss that is about 0.5 dB higher than other generic cases.